FIG. 14(a) shows a structure of a prior art two-wavelength semiconductor laser device recited in, for example "Y. Tokuda et al., Appl. Phys. Lett. 49(24), pp 1629-1631, (1986)". FIG. 14(b) shows the light output vs. current characteristics thereof and FIG. 14(c) shows an energy band diagram in the vicinity of the active layer thereof. In FIG. 14(a), on a p type or n type substrate 701, a p type or n type buffer layer 702, a p type or n type lower cladding layer 703, a p type or n type light confinement layer 704, an undoped quantum well active layer 705, an n type or p type light confinement layer 706, an n type or p type upper cladding layer 707, and an n type or p type contact layer 708 are successively disposed. A SiO.sub.2 film 709 is disposed on a region on the upper cladding layer 707 where the stripe-shaped contact layer 708 is not formed. An electrode 710 is disposed on the rear surface of the substrate 701 and an electrode 711 is disposed on the contact layer 708 and the SiO.sub.2 film 709. Reference numeral 712 designates an impurity diffusion region.
A description is given of the operation.
When a current is injected to the semiconductor laser device, first of all, spontaneous emission light (i in FIG. 14(b)) is emitted. When the current is increased, induced emission arises when the gain owing to the current injection and the total loss in the semiconductor laser are equal to each other (ii in FIG. 14(b)). Then, an oscillation owing to the transition of the quantum well at the bottom state (n=1) is obtained as shown in FIG. 14(c). When current is still further increased, the number of electrons and holes occupying the state of n=2 of the quantum well increases and an oscillation owing to the transition of n=2 shown in FIG. 14(c) is obtained (iii in FIG. 14(b)).
FIG. 8 shows a perspective view of a prior art array type semiconductor laser device which emits a plurality of different wavelengths, recited in, for example Japanese Patent publication No. 61-242093. In FIG. 8, on an n type GaAs substrate 110, an n type GaAs buffer layer 111, an n type Al.sub.x Ga.sub.1-x As (x=0.4) layer 112, an Al.sub.y Ga.sub.1-y As (y=0.2) layer 113, an Al.sub.z Ga.sub.1-z As (z=0.1 to 1) layer 114, an Al.sub.y Ga.sub.1-y As (y=0.2) layer 115, a p type Al.sub.x Ga.sub.1-x As (x=0.4) layer 116 and a p.sup.+ type GaAs layer 117 are successively disposed. Furthermore, a common n side electrode 118 is disposed on the rear surface of the substrate 110 and p side electrodes 119 and 120 are disposed on the p.sup.+ type GaAs layer 117. This array type semiconductor laser device is provided with a first laser light generating region 121 and a second laser light generating region 122, and a reflection coating 125 is disposed on the portion 123a of the first laser light generating region 121 at the device facet 124 while no reflection coating is present on the portion 123b of the second laser light generating region 122 at the device facet 124.
A description is given of the operation of this array type semiconductor laser device.
As discussed above, since the reflection coating 125 is present on the facet 123a of the first laser light generating region 121 while no reflection coating is present on the facet 123b of the second laser light generating region 122, the optical loss in the first laser light generating region 121 is larger than that in the second laser light generating region 122. As a result, an oscillation of wavelength .lambda..sub.1 at quantum level of n=1 occurs in the second laser light generating region 122 having less optical loss while an oscillation of wavelength .lambda..sub.2 at quantum level of n=2 occurs in the first laser light generating region 121 having high optical loss. In this way, the optical loss in the respective laser light generating regions 121 and 122 are changed by changing the reflectivity at the respective laser facets 123a and 123b, whereby a monolithic device oscillating at a plurality of wavelengths is realized.
FIG. 9 shows a cross-sectional view of a prior art semiconductor laser device which emits a plurality of different wavelengths, recited in, for example Japanese Patent Publication No. 63-312688. In FIG. 9, on an n.sup.+ type GaAs substrate 201, an n type Al.sub.z Ga.sub.1-z As cladding layer 202, an n type Al.sub.z Ga.sub.1-z As (z.fwdarw.y) parabolic type diffraction index distribution layer 203, an Al.sub.x Ga.sub.1-x As active layer 204a, an Al.sub.y Ga.sub.1-y As barrier layer 205, an Al.sub.x Ga.sub.1-x As active layer 204b, a p type Al.sub.z Ga.sub.1-z As (z.fwdarw.y) parabolic type diffraction index distribution layer 206 and a p type Al.sub.z Ga.sub.1-z As cladding layer 207 are successively disposed. An n side electrode 211 is disposed on the entirety of the rear surface of substrate 201. Furthermore, a p.sup.+ type GaAs cap layer 208a is disposed on the A region on the cladding layer 207 and a p.sup.+ type GaAs cap layer 208b is disposed on the B region on the cladding layer 207. P side electrodes 212a and 212b are disposed on the cap layers 208a and 208b, respectively. This semiconductor laser device comprises a quantum well active layer part A and a light absorption amount control part B, which controls the oscillation by applying an electric field to the control part B using the electrodes 211 and 212b.
FIG. 10 shows a perspective view of a prior art array type semiconductor device which emits a plurality of different wavelengths, recited in, for example Japanese Patent Publication No. 63-32986. In FIG. 10, on an n type GaAs substrate 305, an n type AlGaAs cladding layer 306, a quantum well active layer 307 and a p type AlGaAs cladding layer 308 are successively disposed, and p type GaAs contact layers 309a, 309b and 309c are disposed on the cladding layer 308 parallel with each other. Insulating films 310 are disposed at regions on the cladding layer 308 where the contact layers 309a, 309b and 309c are not disposed. An n side electrode 303 is disposed on the entirety of the rear surface of substrate 5 and p side electrodes 304a, 304b and 304c each having a different length are disposed on corresponding contact layers 309a, 309b and 309c, respectively. Current terminals 301, 302a, 302b and 302c are respectively connected to the n side electrode 303, p side electrodes 304a, 304b and 304c. In addition, dotted line 311 shows a diffusion front of p type impurities.
It is well known that when a current is injected into the quantum well active layer and the injection carrier density is increased for band filling, as the energy level of the quantum level becomes higher, the gain becomes higher. In a laser array in which each laser element has the same cavity length, the length of the gain region is changed by changing the length of the electrode so that the loss is equivalently changed. When the laser has shorter electrode length, a higher gain is required for oscillation. Accordingly, as the length of the electrode becomes shorter, the energy level required to obtain a laser oscillation becomes higher. In this array type semiconductor laser device, the laser having a shorter electrode for injecting current oscillates at higher quantum level, whereby the oscillation wavelengths of respective lasers can be varied.
FIG. 11 shows a cross-sectional view of a semiconductor quantum well laser device which oscillates at a high quantum level with increasing the cavity loss, recited in, for example Japanese Patent Publication No. 63-54794. In FIG. 11, on an n type GaAs substrate 402, an n type AlGaAs cladding layer 403, a GaAs quantum well active layer 404, a p type AlGaAs cladding layer 405 and a p type GaAs contact layer 406 are successively disposed. An n side electrode 401 is disposed on the rear surface of substrate 402 and a p side electrode 407 is disposed on the contact layer 406 except for the absorption region 409.
In this semiconductor laser device, the absorption region 409 is provided at a part of the device to increase the loss of whole device, thereby enabling oscillation at higher energy level of the quantum well. When the size of this absorption region 409 is adjusted, it is possible to switch the wavelengths from n=1 to n=2 or to output both wavelengths of n=1 and n=2 at the same time by changing the injected current.
FIG. 12 shows a perspective view of a prior art semiconductor laser device in which the current injection electrode is divided into two parts and the current levels injected into the divided electrodes are controlled to enable oscillation at various quantum levels, as recited in Japanese Patent Publication No. 63-32985. In FIG. 12, on an n.sup.+ type GaAs substrate 505, an n type AlGaAs cladding layer 506, a quantum well active layer 507, a p type AlGaAs cladding layer 508 and a p type GaAs contact layer 509 are successively disposed. An n side electrode 503 is disposed on the rear surface of the substrate 505 and p side electrodes 504A and 504B are disposed on the contact layer 509. Current terminals 501, 502A and 502B are connected to these electrodes 503, 504A and 504B, respectively. In addition, dotted line 513 shows a diffusion front of p type impurities.
In this semiconductor laser device, the quantity of injection current from the electrode 504B is changed to control whether the oscillation occurs at the quantum level of n=1 or the quantum level of n=2 in the quantum well active layer 507. That is, when current is not injected into the electrode 504B, oscillation does not occur at a gain for the quantum level of n=1. By increasing the quantity of current injected to the electrode 504A, the gain for the quantum level of n=2 is increased and a laser oscillation occurs at a wavelength corresponding to the quantum level of n=2. When current is supplied to the electrode 504B in this state, the loss inside the laser element decreases and the gain for the quantum level of n=1 exceeds the loss, and then a laser oscillation occurs at a wavelength corresponding to the quantum level of n=1. In this conventional device, the current level supplied to the divided electrode is controlled, whereby oscillations at a plurality of wavelengths are realized.
FIG. 13 shows a prior art semiconductor laser device in which the oscillation wavelength is varied by varying the refractive index of an electro-optic crystal provided at the laser facet, recited in Japanese Patent Publication No. 1-208884. In FIG. 13, a reflecting mirror 602 is provided at one facet of a laser diode 601 and an electro-optic crystal 603 is provided at the other facet. A control circuit 604 for controlling the refractive index of the electro-optic crystal 603 is connected to the electro-optic crystal 603.
In this semiconductor laser device, the oscillation wavelength is determined by the oscillation mode of the laser diode 601 and the oscillation mode of the electro-optic crystal 603. The oscillation mode of the electro-optic crystal 603 can be varied by a voltage is applied to the electro-optic crystal 603 by the control circuit 604 to change the refractive index of the electro-optical crystal 603, whereby the oscillation wavelength of the laser can be varied.
The prior art semiconductor laser devices capable of oscillating at two or more wavelengths are constructed so that the oscillation wavelength is varied by varying the current injected or a wavelength controlling region formed integrally with the laser element so that oscillations occur at different wavelengths in different light emitting regions of the array type laser device. In such a laser device, there is no element separate from the semiconductor laser element to make the semiconductor laser element oscillate at a different wavelength.